The invention relates to a method and apparatus for measuring the elasticity of fluids.
The elasticity of a material is defined in terms of its ability to return to its original shape after being deformed. Certain fluids possess elasticity as well as viscosity. For example, water is viscous and inelastic but shampoo, ink and paint are generally both viscous and elastic.
In many applications, it is desirable to measure the elasticity of a fluid. For example, an ink""s ability to penetrate paper is affected by the ink""s elasticity. A paint""s tendency to drip is related to its elasticity. An evidence of elasticity in a liquid is the xe2x80x9cstringinessxe2x80x9d of the fluid.
A device for measuring extensional properties of fluids is a filament-stretching rheometer. This device stretches a fluid at a constant extensional rate, which means that the length of the filament increases exponentially with time. A filament-stretching rheometer comprises a diameter measuring system, a force transducer, a fluid sample, and an actuator. The fluid is held between an upper and a lower plate, the upper plate being attached to the force transducer, and the lower plate being connected to the actuator.
The actuator moves the lower plate away from the top plate in order to achieve a constant extensional rate while the force transducer measures the force exerted on the filament. Meanwhile, the diameter measuring system provides measurements of the diameter of the fluid filament at its mid point. From the measurements of force and diameter, the stress on the filament can be computed. These measurements may be used to determine the elasticity of the fluid.
However, a filament-stretching rheometer can generally be used only for fluids with relatively high elasticity. The magnitude of the forces incurred when testing liquids having relatively low elasticity, such as shampoo and ink, is much smaller and the resolution of, the noise in, the force transducer make it extremely difficult to obtain accurate measurements of the force on the filament.
It is therefore an object of the present invention to provide a method and apparatus that obviates or mitigates some of the above disadvantages.
The present invention is based upon the recognition that a measurement of the extension of the fluid at the break point may be used as an indication of the elasticity of the fluid.